OK, let's crack this. We're given 90 and our task is to multiply it by 138. Essentially, we'll be adding 90 to itself 138 times.
Step 1: 0 + 90 = 90
Step 2: 90 + 90 = 180
Step 3: 180 + 90 = 270
Step 4: 270 + 90 = 360
Step 5: 360 + 90 = 450
Step 6: 450 + 90 = 540
Step 7: 540 + 90 = 630
Step 8: 630 + 90 = 720
Step 9: 720 + 90 = 810
Step 10: 810 + 90 = 900
Step 11: 900 + 90 = 990
Step 12: 990 + 90 = 1080
Step 13: 1080 + 90 = 1170
Step 14: 1170 + 90 = 1260
Step 15: 1260 + 90 = 1350
Step 16: 1350 + 90 = 1440
Step 17: 1440 + 90 = 1530
Step 18: 1530 + 90 = 1620
Step 19: 1620 + 90 = 1710
Step 20: 1710 + 90 = 1800
Step 21: 1800 + 90 = 1890
Step 22: 1890 + 90 = 1980
Step 23: 1980 + 90 = 2070
Step 24: 2070 + 90 = 2160
Step 25: 2160 + 90 = 2250
Step 26: 2250 + 90 = 2340
Step 27: 2340 + 90 = 2430
Step 28: 2430 + 90 = 2520
Step 29: 2520 + 90 = 2610
Step 30: 2610 + 90 = 2700
Step 31: 2700 + 90 = 2790
Step 32: 2790 + 90 = 2880
Step 33: 2880 + 90 = 2970
Step 34: 2970 + 90 = 3060
Step 35: 3060 + 90 = 3150
Step 36: 3150 + 90 = 3240
Step 37: 3240 + 90 = 3330
Step 38: 3330 + 90 = 3420
Step 39: 3420 + 90 = 3510
Step 40: 3510 + 90 = 3600
Step 41: 3600 + 90 = 3690
Step 42: 3690 + 90 = 3780
Step 43: 3780 + 90 = 3870
Step 44: 3870 + 90 = 3960
Step 45: 3960 + 90 = 4050
Step 46: 4050 + 90 = 4140
Step 47: 4140 + 90 = 4230
Step 48: 4230 + 90 = 4320
Step 49: 4320 + 90 = 4410
Step 50: 4410 + 90 = 4500
Step 51: 4500 + 90 = 4590
Step 52: 4590 + 90 = 4680
Step 53: 4680 + 90 = 4770
Step 54: 4770 + 90 = 4860
Step 55: 4860 + 90 = 4950
Step 56: 4950 + 90 = 5040
Step 57: 5040 + 90 = 5130
Step 58: 5130 + 90 = 5220
Step 59: 5220 + 90 = 5310
Step 60: 5310 + 90 = 5400
Step 61: 5400 + 90 = 5490
Step 62: 5490 + 90 = 5580
Step 63: 5580 + 90 = 5670
Step 64: 5670 + 90 = 5760
Step 65: 5760 + 90 = 5850
Step 66: 5850 + 90 = 5940
Step 67: 5940 + 90 = 6030
Step 68: 6030 + 90 = 6120
Step 69: 6120 + 90 = 6210
Step 70: 6210 + 90 = 6300
Step 71: 6300 + 90 = 6390
Step 72: 6390 + 90 = 6480
Step 73: 6480 + 90 = 6570
Step 74: 6570 + 90 = 6660
Step 75: 6660 + 90 = 6750
Step 76: 6750 + 90 = 6840
Step 77: 6840 + 90 = 6930
Step 78: 6930 + 90 = 7020
Step 79: 7020 + 90 = 7110
Step 80: 7110 + 90 = 7200
Step 81: 7200 + 90 = 7290
Step 82: 7290 + 90 = 7380
Step 83: 7380 + 90 = 7470
Step 84: 7470 + 90 = 7560
Step 85: 7560 + 90 = 7650
Step 86: 7650 + 90 = 7740
Step 87: 7740 + 90 = 7830
Step 88: 7830 + 90 = 7920
Step 89: 7920 + 90 = 8010
Step 90: 8010 + 90 = 8100
Step 91: 8100 + 90 = 8190
Step 92: 8190 + 90 = 8280
Step 93: 8280 + 90 = 8370
Step 94: 8370 + 90 = 8460
Step 95: 8460 + 90 = 8550
Step 96: 8550 + 90 = 8640
Step 97: 8640 + 90 = 8730
Step 98: 8730 + 90 = 8820
Step 99: 8820 + 90 = 8910
Step 100: 8910 + 90 = 9000
Step 101: 9000 + 90 = 9090
Step 102: 9090 + 90 = 9180
Step 103: 9180 + 90 = 9270
Step 104: 9270 + 90 = 9360
Step 105: 9360 + 90 = 9450
Step 106: 9450 + 90 = 9540
Step 107: 9540 + 90 = 9630
Step 108: 9630 + 90 = 9720
Step 109: 9720 + 90 = 9810
Step 110: 9810 + 90 = 9900
Step 111: 9900 + 90 = 9990
Step 112: 9990 + 90 = 10080
Step 113: 10080 + 90 = 10170
Step 114: 10170 + 90 = 10260
Step 115: 10260 + 90 = 10350
Step 116: 10350 + 90 = 10440
Step 117: 10440 + 90 = 10530
Step 118: 10530 + 90 = 10620
Step 119: 10620 + 90 = 10710
Step 120: 10710 + 90 = 10800
Step 121: 10800 + 90 = 10890
Step 122: 10890 + 90 = 10980
Step 123: 10980 + 90 = 11070
Step 124: 11070 + 90 = 11160
Step 125: 11160 + 90 = 11250
Step 126: 11250 + 90 = 11340
Step 127: 11340 + 90 = 11430
Step 128: 11430 + 90 = 11520
Step 129: 11520 + 90 = 11610
Step 130: 11610 + 90 = 11700
Step 131: 11700 + 90 = 11790
Step 132: 11790 + 90 = 11880
Step 133: 11880 + 90 = 11970
Step 134: 11970 + 90 = 12060
Step 135: 12060 + 90 = 12150
Step 136: 12150 + 90 = 12240
Step 137: 12240 + 90 = 12330
Step 138: 12330 + 90 = 12420

So, 90*138 = 12420